1. Field of the Invention
The present invention relates to a method for producing thermoplastic resin films useful for prints, posters, stickers, decorations, etc. More precisely, the invention relates to a method for producing thermoplastic resin films that have good adhesiveness between the coating layer and the film substrate thereof and have good printability and electrostatic resistance.
2. Description Of The Background
In conventional stickers for outdoor advertisement and labels to be stuck to frozen food containers, the coated paper serving as the base of the stickers and labels is poorly resistant to water, and therefore, the surface of the coated paper is further coated with a plastic film for the base for them. Recently, water-resistant thermoplastic films, especially polyolefin-type synthetic papers have become specifically noticed as hopeful label materials that are substitutable for plastic film-coated papers (e.g., JP-B 46-40794 and 49-1782; JP-A-56-118437, 57-12642 and 57-56224).
However, the polyolefin-type synthetic papers are not always satisfactory in point of the printability and the processability thereof since their material, polyolefin is non-polar. In general, therefore, they are subjected to suitable surface treatment before use. For example, one method known for producing polyolefin synthetic papers comprises oxidizing the surface of unstretched films through corona-discharging treatment, then coating the films with a coating liquid, stretching them and optionally further oxidizing their surface through corona-discharging treatment.
JP-A-7-266417 discloses a method for producing laminate resin films, which comprises applying corona treatment of from 30 to 100 w·min/m2 to a film stretched in the machine direction thereof, coating it with a coating liquid of an ethylene-vinyl alcohol copolymer having an ethylene content of from 20 to 45 mol %, drying it, then stretching it inthe cross directionthereof in a tenter, and further applying thereto corona treatment of from30to 100 w·min/m2. However, the laminate resin film produced according to the method is problematic in that the adhesiveness between the substrate and the coating layer thereof is low, and, in addition, it has another problem in point of the safety and sanitation thereof since the coating liquid for it contains an organic solvent. Accordingly, it is desired to improve the film in these points.
JP-A-11-342534 discloses a method of surface treatment of thermoplastic resin films, which comprises oxidizing a thermoplastic resin film, then coating it with a coating liquid of polyethylenimine, and stretching it. The method solves the problem of safety and sanitation since the coating liquid used is an aqueous solution, but has another problem in that its printing compatibility is not good because of the probability that the adhesiveness between the film substrate and the coating layer may be low. Therefore, it is desired to improve the method in this point.
An object of the present invention is to provide a method for producing thermoplastic resin films that have good adhesiveness between the film substrate and the coating layer thereof and have good printability and electrostatic resistance.